True Mode is a higher difficulty mode activated with the Sigil of the Corrupted One.
The sigil only drops once per world, obtained from the Moon Lord in expert mode.
The item can only be used in expert mode and while there is no boss alive.
Attempting to use it outside of these conditions does nothing and does not consume the item.
The mode changes the behavior of the mod's post-Moon Lord bosses, and allows access to a few new items and mechanics.
Grants immunity to knockback, a large number of debuffs, turns the player into a Werewolf at night and a Merfolk in water, increases all stats by a small amount, absorbs 25% of damage done to players on your team (if above 25% life), increases the player's max life by 100, and has a 10% chance of blocking a hostile projectile that hits the player.